If you already have a skirt style in mind, browse through the major pattern companies for something similar to your style. Look at the line drawings, not the photos. The Big Four all have retro patterns.

I know I don't like to put a lot of time, energy and money into a costume that will be worn once or maybe twice. Party costumes don't need to be 100% authentic in construction details or even designs. All a party costume has to do is capture the flavor or essence of an era. I won first place at a 20's themed New Years Party wearing a black 70's disco dress! The key to that costume was the wig, jewelry and accessories.

Costume for historical recreations or associations are different. The rules for these may require use of specific design or construction elements.

idk when your party is, but you can find 60s skirt patterns that are pretty well priced and you can find the style and such you want. Some patterns on Etsy are $3 or less (so usually $5 after shipping?). Or eBay too.

Also, I attack antique malls and antique stores for vintage patterns, and usually snatch them up for $1-5 each. I have quite the stash of vintage patterns, and while the sizing is very different from my modern figure, I've made quite a few dresses, pants, skirts, and tops for myself as well as my poor, unsuspecting children. It's a good exercise in alterations, at least!